1. Field of the Invention
The present invention relates to data communications. More specifically, the present invention relates to improved data frame synchronization in data communication systems.
2. Description of the Related Art
Modern communication systems frequently employ data as a means of communicating information. Data is useful for the purpose of communicating audio, data, and control information. When data is employed as the information medium, it must be arranged into a format that is consistently applied at both the transmitting and receiving ends of the system so that orderly and efficient communications are possible. A common data formatting approach is TDMA (Time Division Multiple Access) which uses TDM (Time Division Multiplexing) of the data bits. Data bits are arranged into data frames of predetermined length and are sent from a transmitter to one or more receivers. The physical medium can be radio waves, metallic conductors, or fiber optic conductors. Each receiver must synchronize to the received frames so that the data can be decoded in a way that is predetermined for the particular system and protocol. Typically, such systems use synchronization symbols, which are groups of data bits, as a reference to each data frame for accurately locating the data frame in time.
In operation, receivers that are first activated must align themselves to the data frames so that each can be ready to communicate when needed. Re-alignment needs to occur when a receiver loses alignment for some other reason. Alignment involves finding the carrier signal, locating the data pulses therein, and identifying the synchronization symbol within the data frames. In some systems, where two-way communications are employed, the receivers may need to reply to the transmitters in some fashion. In the case of a radio communications system, radio units may need to check-in to identify their presence and availability for communications. In any event, the process of a receiver aligning to the data frames of a communications system involves some time, which defines a period of unavailability for communications. Naturally, it is preferable to keep the time of unavailability to a minimum.
All communication systems are subject to the detrimental effects of noise. The greater the amount of noise present, relative to the data signal strength, the more problematic the noise is to efficient and reliable communications. This relationship is defined as a signal to noise ratio (or “SNR”). When the SNR becomes very low, a receiver will no longer be able to accurately decode data. When there is a short period of poor SNR, then a receiver may lose some data. Most communications protocols deal with this problem by employing error correction or error detection techniques. By using these, errors in communications can be either corrected, or data can be retransmitted to overcome the errors caused by noise or other factors. When the noise affects the ability of a receiver to maintain alignment with the data frames, the problem is more severe because extended periods of unavailability for communications may result while the receiver attempts to re-align itself.
Thus, the effects of noise, while correctable for short periods of poor SNR, have a greater detrimental effect in the case where the noise affects a receiver's ability to maintain alignment with data frames because the time to reacquire alignment to the data frames is a period when data communications are not possible. Therefore, there is a need in the art to improve the ability of data receivers to achieve and maintain data frame alignment in the presence of poor signal quality.